THF: new railway I^KTWEEN UPMINSTER and ROMFORD. 13 



date than the Chalky Boulder Clay ; the only standard of age that 

 can be taken without introducing confusion.'' 



On the other hand, I pointed out that the absence of definite, 

 traceable terraces was invariably to be found where a river cuts 

 through soft, incoherent strata ; and that it implied no peculiarity 

 in the formation of the river valley. That the general evidence all 

 pointed towards the view that the highest terrace was in this case, 

 as usual, the oldest ; and that, in default of special evidence point- 

 ing yet more strongly another way, the only rational course was to 

 hold this view. That the low level of the Hornchurch Boulder Clay 

 necessarily implied merely deposition in a narrow valley, or hollow 

 with a direction at right angles to that of the present Thames. 

 That there may have been at the time a valley, possibly coinciding 

 here and there, more or less, with that of the Thames ; but that we 

 had no evidence of it and its deposits, the present valley and its 

 beds being post-Glacial. And that the Essex Boulder Clay lies on 

 a very uneven surface, and at various levels. 



In short, I maintain that the evidence now available clearly 

 points to the conclusion that the River Drift of the lower Thames, 

 with the Mammoth and other remains contained therein, is post- 

 Glacial. Absolute proof is, of course, unattainable. But it seems 

 to me that this conclusion can be discredited only by the discovery 

 of some special evidence pointing yet more strongly in another 

 direction. And of this we have, at present, no sign. 



Notes. 



A most interesting example of the occasional coincidence and 

 frequent variation between the pre-Glacial and post-Glacial channels 

 of an English river, is given by Sir Andrew Ramsay in his "Physical 

 Geology and Cieography of Great Britain" (edit. 5, p. 531). It is 

 that of the Wear, in Durham. A letter from Mr. H. H. Howell 

 (who superintended the Geological Survey of the Newcastle coalfield) 

 to Sir A. Ramsay thus describes the changes that occurred : — 



" The pre-Glacial valley runs nearly north and south from 

 Durham to Newcastle. The river Wear, instead of following this 

 old valley, meanders about, winding in and out of it, and at Durham 

 cutting right across it, and passing into the sandstones of the Coal- 

 measures, through which it has cut its way in a narrow gorge. At 

 Chester-le-Street, half-way between Durham and Newcastle, the river 



6 Of coins;- I bj- no means assert that the Mammoth is necessarily to be found only in post- 

 Glacial Beds. I see no reason why it should not be pre-Glacial as well. Each case must be 

 decided on iis own evidence. 



